Acceleration sensor having resonators with reduced height

ABSTRACT

A compact, highly sensitive acceleration sensor that is not affected by factors other than acceleration, such as a change in temperature, includes a bimorph acceleration-sensor element including first and second resonators and attached to opposite sides of a base plate with respect to a direction in which acceleration is applied. One longitudinal end or both longitudinal ends of the acceleration-sensor element is/are fixed such that the resonators bend in the same direction in response to the acceleration. Changes in frequency or changes in impedance in the resonators caused by the bending of the acceleration-sensor element are differentially detected in order to detect the acceleration. The acceleration-sensor element is bendable about a central bending plane N 1  in response to the acceleration, the central bending plane N 1  being positioned at a central portion of the base plate with respect to the application direction of acceleration. Electrodes are disposed on main surfaces of the resonators, the main surfaces being substantially perpendicular to the application direction of acceleration. The height H 1  of the resonators in a direction that is substantially perpendicular to the application direction of acceleration is smaller than the height H 2  of the base plate in the direction that is substantially perpendicular to the application direction of acceleration.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to acceleration sensors, and particularly,to an acceleration sensor including a piezoelectric material.

2. Description of the Related Art

A known acceleration sensor including piezoelectric ceramics is, forexample, disclosed in Japanese Patent No. 2780594, hereinafter referredto as Patent Document 1. Such an acceleration sensor is provided with abimorph sensor element including a pair of piezoelectric units which arecomposed of piezoelectric ceramics and are integrally joined to eachother in an opposing manner. The sensor element is held inside a casingin a double-supported fashion. When acceleration is applied to theacceleration sensor, the sensor element bends, thus generating stress inthe piezoelectric units. The electric charge or voltage generated due tothe piezoelectric effect is then detected in order to determine theacceleration. Acceleration sensors of this type are advantageous in viewof their compactness and their capability of being formed easily intosurface-mounted units (chip units).

In an acceleration sensor based on the above-described principle, a biascurrent flowing from a circuit is stored in a capacitor C of thepiezoelectric material. In order to prevent the circuit from becomingsaturated, a resistor R is required for allowing the bias current to bereleased. However, since the resistor R and the capacitor C define ahigh pass filter, the acceleration in the frequencies below the cut-offlevel, such as DC and low frequency, cannot be detected.

On the other hand, an acceleration sensor disclosed in JapaneseUnexamined Patent Application Publication No. 2002-107372, hereinafterreferred to as Patent Document 2, particularly, the acceleration sensorshown in FIG. 8 in Patent Document 2, includes a single base plate whoseopposite sides respectively have first and second resonators formed of apiezoelectric material attached thereto so as to define anacceleration-sensor element, each of the first and second resonatorshaving electrodes on opposite sides thereof. One longitudinal end orboth longitudinal ends of the acceleration-sensor element is/are fixedsuch that the first and second resonators are bendable in their opposingdirection in response to acceleration. When the acceleration-sensorelement bends in response to acceleration, changes in frequency orchanges in impedance in the first and second resonators caused by thebending of the acceleration-sensor element are differentially detectedin order to detect the acceleration.

In this case, the acceleration in a DC or low-frequency level can bedetected. Moreover, the changes in frequency or the changes in impedancein the two resonators are differentially detected instead of beingdetected in a separate manner. This counterbalances the stress (forexample, a stress caused by a change in temperature) applied to bothresonators. Thus, a high-sensitivity acceleration sensor, which isunaffected by, for example, a change in temperature, is achieved.Furthermore, because the central bending plane (i.e. a plane wherestress is 0) is set in the base plate, a large degree of tensile stressand compressive stress can be generated in the resonators disposed onthe opposite sides of the base plate. Accordingly, this improves thesensitivity of the sensor.

However, because the first and second resonators and the base plate havethe same height, namely, the same dimension in a direction perpendicularto the direction in which the acceleration is applied, the first andsecond resonators have a large cross-sectional area. This means that thetensile stress and the compressive stress generated in the resonators inresponse to the acceleration cannot be increased. Consequently, thisprevents further improvement of the sensitivity (S/N ratio).

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a compact, high-sensitivityacceleration sensor that is prevented from being affected by factorsother than acceleration, such as a change in temperature or otherfactors.

According to a preferred embodiment of the present invention, anacceleration sensor includes a base plate, and first and secondresonators each formed of a piezoelectric material and each havingelectrodes on two opposite main surfaces thereof, each resonator havinga vibrating section at an intermediate portion of the resonator withrespect to the longitudinal direction thereof. The first and secondresonators are attached to opposite sides of the base plate with respectto a direction in which acceleration is applied so as to define abimorph acceleration-sensor element, wherein one longitudinal end orboth longitudinal ends of the acceleration-sensor element is/are fixedsuch that the first and second resonators bend in the same direction inresponse to the acceleration, and wherein changes in frequency orchanges in impedance in the first and second resonators caused by thebending of the acceleration-sensor element are differentially detectedin order to detect the acceleration. The acceleration-sensor element isbendable about a central bending plane in response to the acceleration,the central bending plane being positioned at a central portion of thebase plate with respect to the application direction of acceleration.Each of the first and second resonators is attached to the base platesuch that the opposite main surfaces having the electrodes disposedthereon are substantially perpendicular to the application direction ofacceleration. A height of the first and second resonators in a directionthat is substantially perpendicular to the application direction ofacceleration is smaller than a height of the base plate in the directionthat is substantially perpendicular to the application direction ofacceleration.

According to the first preferred embodiment of the present invention,the acceleration-sensor element has a bimorph structure in which theresonators are attached to the opposite sides of the single base plate,and the central bending plane is positioned at the central portion ofthe base plate with respect to the thickness of the base plate.Consequently, when acceleration is applied to the acceleration-sensorelement, the base plate functions as a mass body so as to effectivelygenerate a tensile stress in one resonator and a compressive stress inthe other resonator. In a certain vibration mode, the frequency in theresonator with tensile stress decreases while the frequency in theresonator with compressive stress increases. By differentially detectingthe changes in frequency or the changes in impedance in the resonators,the acceleration can be detected. Moreover, since the changes infrequency or the changes in impedance in the two resonators are detectedin a differential manner, the stress applied to both resonators (forexample, a stress caused by a change in temperature) can becounterbalanced. Accordingly, a high-sensitivity acceleration sensorthat is unaffected by, for example, a temperature change is provided.

One of the unique aspects of preferred embodiments of the presentinvention is that the electrodes of the first and second resonators aredisposed on the main surfaces that are substantially perpendicular tothe application direction of acceleration. This allows the resonators tohave a predetermined thickness, or in other words, this allows theopposing electrodes of each pair to be separated from each other by apredetermined distance so that the resonance frequency of the resonatorsbecomes constant. Consequently, a common signal-processing circuit isachieved.

In addition, the height of the first and second resonators in thedirection that is substantially perpendicular to the applicationdirection of acceleration is smaller than that of the base plate. Inother words, this reduces the cross-sectional area of the first andsecond resonators, and thus allows the tensile stress and thecompressive stress generated in the resonators in response to theacceleration to be increased. Accordingly, this improves the sensitivity(S/N ratio) for detecting the acceleration.

There are, for example, two approaches for obtaining a signal that isproportional to the acceleration acting upon the acceleration-sensorelement based on the signals that are differentially detected from thetwo resonators. One approach is to oscillate the first and secondresonators separately with different frequencies, determine theoscillating-frequency difference, and obtain the signal that isproportional to the acceleration based on the frequency difference. Theother approach is to oscillate the first and second resonators with thesame frequency, detect the phase difference or the oscillationdifference based on the difference in electric impedance between theresonators, and obtain the signal that is proportional to theacceleration based on the phase difference or the oscillationdifference.

Furthermore, the first and second resonators are preferably attached tothe opposite sides of the base plate at positions where the first andsecond resonators are opposed to each other.

Although it is possible to attach the two resonators to the oppositesides of the base plate at positions where the two resonators do notoppose each other, such a structure may lead to detection errors. Thisis due to the fact that if the acceleration-sensor element bends inresponse to an external force applied from a direction other than theapplication direction of acceleration (off-axis bending), the tworesonators may generate different signals. In contrast, by attaching thetwo resonators to the opposite sides of the base plate at positionswhere the two resonators are opposed to each other, signals can bedetected from the two resonators in a differential manner. Thus, thedifference in detection with respect to the off-axis bending can becompensated for.

Furthermore, each of the first and second resonators is preferablyattached to a central portion of the base plate with respect to a heightdirection of the base plate, the height direction being substantiallyperpendicular to the application direction of acceleration.

Consequently, in addition to being attached to the opposite sides of thebase plate at positions where the two resonators are opposed to eachother, each resonator may be attached to the central portion of the baseplate with respect to the height direction. This structure can furthercompensate for the difference in detection since no stress acts upon thetwo resonators in response to off-axis bending.

Furthermore, the base plate and the first and second resonators arepreferably formed of at least one material having substantially the samecoefficient of thermal expansion.

If the coefficient of thermal expansion differs significantly betweenthe base plate and the first and second resonators, a tensile stress ora compressive stress may be generated in the resonators due to a changein temperature in the environment even when no acceleration is applied.This leads to changes in frequency or changes in impedance. By allowingthe base plate and the first and second resonators to have substantiallythe same coefficient of thermal expansion, the temperature drift relatedto the output from the sensor can be prevented, thus reducing thermalhysteresis.

The base plate and the first and second resonators may be formed of thesame material, or may be formed of different materials. The coefficientof thermal expansion between the base plate and the resonators may bedifferent to an extent such that the changes in frequency or the changesin impedance in the resonators in an operating temperature limit arewithin an error range and are thus significantly small.

Furthermore, opposite outer surfaces of the acceleration-sensor elementmay be respectively fixedly supported by a pair of casing components atthe longitudinal end of the acceleration-sensor element, the outersurfaces being opposite to each other with respect to the applicationdirection of acceleration. Moreover, open planes defined by theacceleration-sensor element and the casing components are covered with apair of cover components so that a displacement portion of theacceleration-sensor element, which is bendable in response to theacceleration, is disposed within an enclosed space. Such a packagedstructure allows the displacement portion to be blocked from theoutside, whereby a surface-mounted unit that is prevented from beingaffected by, for example, moisture and dust is provided.

Furthermore, one of the electrodes in each of the first and secondresonators is preferably disposed at a free-end side of the resonatorand is preferably connected with a common electrode via an extractionelectrode provided on the base plate, the common electrode beingprovided at a fixed-end side of an outer surface of a combination of thecasing components and the cover components. Moreover, the otherelectrode in the first resonator is preferably disposed at a base-endside of the first resonator, the electrode being connected with a firstindependent electrode provided at a free-end side of the outer surfaceof the combination of the casing components and the cover components,the electrode being connected with the first independent electrode via afirst extraction electrode provided on one of the casing components. Theother electrode in the second resonator is preferably disposed at abase-end side of the second resonator, the electrode being connectedwith a second independent electrode provided at the free-end side of theouter surface of the combination of the casing components and the covercomponents, the electrode being connected with the second independentelectrode via a second extraction electrode provided on the other casingcomponent.

When using an acceleration-sensor element of a cantilever structure,three electrodes are concentrated at the base-end portion of theacceleration-sensor element, and for this reason, it is difficult to setthese electrodes to be distant from one another on the outer surface ofthe package. In order to set the three external electrodes distant fromone another, one pair of the electrodes from the two resonators isconnected to the common electrode, provided at the fixed-end side of theouter surface of the package (the combination of the casing componentsand the cover components), via the base plate, and the other pair of thetwo remaining electrodes is respectively connected to two independentelectrodes, provided at a side of the outer surface opposite to thefixed-end side of the package, via the casing components. Accordingly,when used as a surface-mounted unit, a short circuit is prevented fromoccurring among the electrodes.

Furthermore, the casing components are preferably provided with aplurality of internal electrodes disposed on upper surfaces of thecasing components, the internal electrodes being connected with theelectrodes in each of the first and second resonators.

In this case, the characteristics of the resonators can be measuredeasily by allowing measuring terminals to come into contact with theinternal electrodes disposed on the upper surfaces of the casingcomponents.

Accordingly, preferred embodiments of the present invention provide anacceleration-sensor element having a bimorph structure in whichresonators are attached to opposite sides of a base plate. Whenacceleration is applied to the acceleration-sensor element, changes infrequency or changes in impedance in the resonators are detected in adifferential manner. Accordingly, a high-sensitivity acceleration sensorthat is unaffected by, for example, a temperature change is provided.

Furthermore, since the electrodes of the first and second resonators aredisposed on the main surfaces that are substantially perpendicular tothe application direction of acceleration, the opposing electrodes ofeach resonator are separated from each other by a predetermineddistance. This allows the resonance frequency of the resonators to beconstant, whereby a common signal-processing circuit is achieved.

Furthermore, since the height of the first and second resonators in thedirection that is substantially perpendicular to the applicationdirection of acceleration is smaller than that of the base plate, thetensile stress and the compressive stress generated in the resonators inresponse to the acceleration are increased, whereby the sensitivity (S/Nratio) is improved.

These and other features, elements, characteristics and advantages ofthe present invention will become more apparent from the followingdetailed description of preferred embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of an acceleration sensoraccording to a preferred embodiment of the present invention.

FIG. 2 is an exploded perspective view of the acceleration sensor shownin FIG. 1.

FIG. 3 is an exploded perspective view of an acceleration-sensor elementprovided in the acceleration sensor shown in FIG. 1.

FIG. 4 is a plan view of the acceleration sensor shown in FIG. 1 in astate where cover components of the acceleration sensor are removed.

FIG. 5 includes perspective views illustrating a method for cutting amaster substrate into segments in order to form resonators.

FIG. 6 is a circuit diagram of an example of an acceleration sensordevice provided with the acceleration sensor according to a preferredembodiment of the present invention.

FIG. 7 is circuit diagram of another example of an acceleration sensordevice provided with the acceleration sensor according to a preferredembodiment of the present invention.

FIG. 8 is an exploded perspective view of an acceleration sensoraccording to another preferred embodiment of the present invention.

FIG. 9 is an exploded perspective view of the acceleration sensor shownin FIG. 8 in a state where cover components of the acceleration sensorare removed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described.

First Preferred Embodiment

FIGS. 1 to 5 illustrate an acceleration sensor according to a firstpreferred embodiment of the present invention.

An acceleration sensor 1A includes a bimorph acceleration-sensor element2A supported in a cantilever manner by a pair of insulative casingcomponents 6 and a pair of insulative cover components 7 composed of,for example, insulative ceramics. Referring to FIGS. 2 and 3, if thedirection in which acceleration G is applied is defined as the y-axisdirection, the longitudinal and height directions of theacceleration-sensor element 2A are defined as the x-axis direction andthe z-axis direction, respectively.

The acceleration-sensor element 2A in the first preferred embodimentincludes resonators 3 and 4 which are integrally attached to tworespective opposite sides of a base plate 5 with respect to theapplication direction of acceleration (y-axis direction) viacorresponding spacers 51 to 54. The resonators 3 and 4 are resonators ofan energy-trap thickness-shear vibration type and each include apiezoelectric ceramic plate strip. The resonators 3 and 4 respectivelyinclude a pair of electrodes 3 a and 3 b and a pair of electrodes 4 aand 4 b. The electrodes 3 a and 3 b are disposed on main surfaces of thepiezoelectric ceramic plate strip of the resonator 3, and the electrodes4 a and 4 b are disposed on main surfaces of the piezoelectric ceramicplate strip of the resonator 4, the main surfaces being substantiallyperpendicular to the application direction of acceleration G. One set ofthe electrodes 3 a and 4 a of the resonators 3 and 4 is exposed at theouter side surfaces of the acceleration-sensor element 2A, whereas theother set of the electrodes 3 b and 4 b faces the base plate 5. Afirst-end portion of the electrode 3 a on the outer surface of theresonator 3 is opposed to a second-end portion of the electrode 3 b onthe inner surface at an intermediate portion of the resonator 3 withrespect to the longitudinal direction thereof. Similarly, a first-endportion of the electrode 4 a on the outer surface of the resonator 4 isopposed to a second-end portion of the electrode 4 b on the innersurface at an intermediate portion of the resonator 4 with respect tothe longitudinal direction thereof. On the other hand, the second-endportion of the electrode 3 a and the first-end portion of the electrode3 b extend away from each other towards the opposite ends of theresonator 3, and similarly, the second-end portion of the electrode 4 aand the first-end portion of the electrode 4 b extend away from eachother towards the opposite ends of the resonator 4. Here, the second-endportions of the electrodes 3 a and 4 a do not completely extend to thecorresponding ends of the respective resonators 3 and 4. The resonators3 and 4 have the same height H₁ in the z-axis direction, and have thesame thickness T₁ in the y-axis direction. Thus, the resonators 3 and 4have the same resonance frequency when no acceleration is being appliedthereto. Since the height H₁ of the resonators 3 and 4 is smaller than aheight H₂ of the base plate 5 in the z-axis direction, the stressgenerated in the resonators 3 and 4 due to acceleration applied to theresonators 3 and 4 is greater than in a case where H₁=H₂. In the firstpreferred embodiment, H₁ is preferably set at about ⅕ or less of H₂.

As shown in FIG. 5, the resonators 3 and 4 are preferably formed bycutting a single master piezoelectric substrate M into segments, andpairing adjacent cut segments so as to form pairs of resonators. Thisreduces the difference in the resonance characteristics including thetemperature characteristics between the resonators of each pair.Accordingly, the difference in the output signal between the tworesonators, which may be caused by a change in temperature, is reducedso as to achieve an acceleration sensor having less output fluctuation.

The upper and lower main surfaces of the resonator 3 are provided withspacers 31 and 32 preferably having substantially the same thickness T₁as the resonator 3. The spacers 31 and 32 are fixed adjacent to tworespective opposite ends of the resonator 3 with respect to thelongitudinal direction of the resonator 3. Similarly, the upper andlower main surfaces of the resonator 4 are provided with spacers 41 and42 having the same thickness T₁ as the resonator 4. The spacers 41 and42 are fixed adjacent to two respective opposite ends of the resonator 4with respect to the longitudinal direction of the resonator 4. An areawhere the electrodes 3 a and 3 b are opposed to each other and an areawhere the electrodes 4 a and 4 b are opposed to each other definevibrating sections. In detail, each vibrating section is disposed wherethe pairs of spacers 31 and 32 or the pairs of spacers 41 and 42 are notdisposed. In the first preferred embodiment, the spacers 32 and 42disposed adjacent to free ends of the respective resonators 3 and 4 havea greater length than the spacers 31 and 41 disposed adjacent to baseends of the respective resonators 3 and 4. For this reason, thevibrating sections of the resonators 3 and 4 are disposed close to thebase end, i.e. a fixed end, of the acceleration-sensor element 2A.Because a stress generated in response to acceleration is greatertowards the base end of a cantilever structure, providing the vibratingsections close to the base ends of the resonators 3 and 4 allows theresonators to receive a greater stress, thus improving the sensitivityof the sensor. The height of the combination of the resonator 3 and thespacers 31 or 32 and the height of the combination of the resonator 4and the spacers 41 or 42 are preferably substantially equal to theheight H₂ of the base plate 5.

Alternatively, the spacers 31, 32, 41, and 42 may be omitted such thatthe resonators 3 and 4 are directly attached to the two respectiveopposite sides of the base plate 5.

The resonators 3 and 4 are respectively attached to positions on the twoopposite sides of the base plate 5 where the resonators 3 and 4 areopposed to each other, and particularly, are most preferably attached tothe central portions of the base plate 5 with respect to the heightdirection of the base plate 5. This is due to the fact that even if theacceleration-sensor element were to bend in response to an externalforce from a direction other than the direction in which theacceleration is applied (off-axis bending), the difference in detectionwith respect to the off-axis bending can be compensated for by receivingsignals from the two resonators 3 and 4 in a differential manner. Thedetection difference between the two resonators 3 and 4 opposed to eachother is reduced due to the fact that, even in the case of off-axisbending, the same amount of stress acts upon the two resonators. Inparticular, attaching the two resonators 3 and 4 to the centralpositions of the base plate 5 with respect to the height direction ofthe base plate 5 further reduces the detection difference. Specifically,this is due to the fact that even when stress is generated in theresonators 3 and 4 due to off-axis bending, since each of the resonators3 and 4 bends about a central bending plane with respect to the heightdirection, the stress is counterbalanced within the resonator 3 or 4.

One side surface of the combination of the resonator 3 and the spacers31 with respect to the y-axis direction is provided with a connectionelectrode 33 connected with the electrode 3 a of the resonator 3 andextending continuously across the side surface in the height direction(z-axis direction). Similarly, the other side surface of the combinationof the resonator 3 and the spacers 32 with respect to the y-axisdirection is provided with a connection electrode 34 connected with theelectrode 3 b of the resonator 3 and extending continuously across theside surface in the height direction (z-axis direction). On the otherhand, one side surface of the combination of the resonator 4 and thespacers 41 with respect to the y-axis direction is provided with aconnection electrode 43 connected with the electrode 4 a of theresonator 4 and extending continuously across the side surface in theheight direction (z-axis direction). Similarly, the other side surfaceof the combination of the resonator 4 and the spacers 42 with respect tothe y-axis direction is provided with a connection electrode 44connected with the electrode 4 b of the resonator 4 and extendingcontinuously across the side surface in the height direction (z-axisdirection). Specifically, the connection electrodes 33 and 43 disposedclose to the base ends of the resonators 3 and 4, respectively, aredisposed on the outer side surface of the combination of the resonator 3and the spacers 31 and the outer side surface of the combination of theresonator 4 and the spacers 41. Thus, the connection electrode 33 issurface-connected with the electrode 3 a, and the connection electrode34 is surface-connected with the electrode 3 b. Similarly, theconnection electrode 43 is surface-connected with the electrode 4 a, andthe connection electrode 44 is surface-connected with the electrode 4 b.Accordingly, this ensures the electrical connections among thecomponents.

The base plate 5 is an insulative plate having the same length as theresonators 3 and 4, and is bendable with respect to a central bendingplane (indicated by a dashed line N1 in FIG. 4) in response toacceleration G applied to the acceleration-sensor element 2A. Thecentral bending plane is positioned at the central portion of the baseplate 5 with respect to the thickness direction (y-axis direction) ofthe base plate 5. The base plate 5 and each of the resonators 3 and 4have a gap 5 a therebetween (see FIG. 4) which has a wider dimensionthan the range in which the resonator 3 or 4 vibrates in an enclosedmanner. In the first preferred embodiment, although the spacers 51 to 54are attached to the corresponding sides of the base plate 5 and areseparated by a predetermined distance in the longitudinal direction ofthe base plate 5 in order to form the gaps 5 a, the opposite sides ofthe base plate 5 may alternatively be provided with depressions in placeof the spacers. As a further alternative, the base plate 5 and each ofthe resonators 3 and 4 may have an adhesive layer therebetween havingenough thickness for forming gaps.

The spacers 51 and 52 disposed adjacent to the base end preferably havethe same length as the spacers 31 and 41 disposed adjacent to the baseends of the respective resonators 3 and 4. Moreover, the height of thespacers 51 and 52 (in the z-axis direction) is preferably substantiallythe same as the height H₂ of the base plate 5. Similarly, the spacers 53and 54 disposed adjacent to the free end preferably have substantiallythe same length as the spacers 32 and 42 disposed adjacent to the freeends of the respective resonators 3 and 4. Moreover, the height of thespacers 53 and 54 (in the z-axis direction) is preferably substantiallythe same as the height H₂ of the base plate 5.

The resonators 3 and 4, the spacers 31, 32, 41, and 42, the base plate5, the spacers 51 to 54 define the acceleration-sensor element 2A andare preferably composed of materials having the same coefficient ofthermal expansion as that of the resonators 3 and 4 (for example, aceramic material such as PZT). This prevents stress from being generatedin the resonators 3 and 4 due to differences in thermal expansion causedby a change in temperature.

One side surface of the base plate 5 having the spacers 51 and 53attached thereon is provided with an extraction electrode 5 b extendingover the entire length of the side surface. The extraction electrode 5 bis connected with an internal electrode 61 extending continuously acrossthe top surface of the base-end portion of the acceleration-sensorelement 2A when the resonators 3 and 4 are in a combined state. On theother hand, an internal electrode 64 extends continuously across afree-end portion of the top surface of the combination of the base plate5 and the spacers 53, 54, 32, and 42. The internal electrode 64functions as a connector for interconnecting the extraction electrode 5b disposed on one side surface of the base plate 5 with the connectionelectrodes 34 and 44 disposed on the side surfaces of the respectiveresonators 3 and 4.

The two opposite sides of the acceleration-sensor element 2A withrespect to the application direction of acceleration G are respectivelycovered with the pair of left and right casing components 6. Each casingcomponent 6 is substantially U-shaped in cross section, and a projection6 a disposed adjacent to a first end of the casing component 6 isattached to the base-end portion of one of the opposite side surfaces ofthe acceleration-sensor element 2A. On the other hand, a projection 6 bdisposed adjacent to a second end of one casing component 6 is attachedto a projection 6 b of the other casing component 6 via a spacer 2 adisposed therebetween. The spacer 2 a according to the first preferredembodiment is preferably formed by cutting a longitudinal end-segment ofthe acceleration-sensor element 2A, and includes portions of the baseplate 5, the resonators 3 and 4, and the spacers 53, 54, 32, and 42. Theprojections 6 a and 6 b of each casing component 6 have a depression 6 cdisposed therebetween, which is a space where the acceleration-sensorelement 2A is allowed to bend into. Moreover, each casing component 6 ispreferably provided with a stopper 6 d disposed near an inner side ofthe second-end projection 6 b. The stopper 6 d restricts anover-displacement of the acceleration-sensor element 2A when a largeamount of acceleration G is applied so as to prevent theacceleration-sensor element 2A from being deformed or damaging. If thedegree of bending of the acceleration-sensor element 2A is extremelysmall and the bending spaces can thus be formed based on the thicknessof adhesive layers between the casing components 6 and theacceleration-sensor element 2A, the depressions 6 c and the stoppers 6 dmay be omitted.

The inner side surface and the top surface of one casing component 6 arerespectively provided with extraction electrodes 62 a and 62 b which areconnected with each other, and the inner side surface and the topsurface of the other casing component 6 are respectively provided withextraction electrodes 63 a and 63 b which are connected with each other.The casing components 6 are joined with the acceleration-sensor element2A via an electrically conductive adhesive for allowing the electrodes33 and 62 a to be electrically connected with each other, and theelectrodes 43 and 63 a to be electrically connected with each other. Inthis case, an anisotropic electrically-conductive adhesive is preferablyused in order to prevent a short circuit between the internal electrode61, extending continuously across the base-end portion of the topsurface of the combination of the casing components 6 and theacceleration-sensor element 2A, and an external electrode 71, andbetween the electrode 33 and the electrode 43.

The extraction electrodes 62 b and 63 b disposed on the top surfaces ofthe corresponding casing components 6 are aligned with the internalelectrode 64 disposed on the free-end portion of the top surface of theacceleration-sensor element 2A. The electrodes 62 b, 63 b, and 64 areformed after the casing components 6 are attached to theacceleration-sensor element 2A, and can be fabricated simultaneously byperforming, for example, a sputtering process or a deposition process onthe top surface of the combination of the casing components 6 and theacceleration-sensor element 2A. In this case, the internal electrode 61can also be formed at the same time.

The upper and lower open planes of the combination of theacceleration-sensor element 2A and the casing components 6 arerespectively covered with the pair of upper and lower cover components7. The inner surface of each cover component 7 is provided with acavity-forming recess 7 a for preventing the acceleration-sensor element2A from coming into contact with the cover component 7. A peripheralregion surrounding the recess 7 a is attached to one of the open planes.For this reason, a portion of the acceleration-sensor element 2A to bedisplaced in response to acceleration G is completely enclosed by thecasing components 6 and the cover components 7. Similar to the casingcomponents 6, the cavity-forming recess 7 a in the inner surface of eachcover component 7 may be omitted if the cavity can be formed based onthe thickness of an adhesive layer provided along the frame region onthe inner surface of the cover component 7.

The outer surface of each cover component 7 is provided with a portionof an external electrode 71 positioned adjacent to the base end of theacceleration-sensor element 2A, and portions of two external electrodes72 and 73 positioned close to the free end of the acceleration-sensorelement 2A. Referring to FIG. 1, the two external electrodes 72 and 73are positioned distant from the external electrode 71 in thelongitudinal direction (x-axis direction), and moreover, are disposed ontwo opposite sides from each other in the application direction ofacceleration (y-axis direction). The positioning of the two externalelectrodes 72 and 73 is not limited to that shown in FIG. 1.Alternatively, the external electrodes 72 and 73 may be disposed at anend opposite to the end at which the external electrode 71 is disposed,such that the two electrodes 72 and 73 are disposed on opposite sides inthe y-axis direction at that end.

The acceleration sensor 1A having the structure described above has thefollowing conductive path.

Specifically, the electrode 3 a of the resonator 3 is connected with theexternal electrode 72 via the connection electrode 33 and the extractionelectrodes 62 a and 62 b. On the other hand, the electrode 4 a of theresonator 4 is connected with the external electrode 73 via theconnection electrode 43 and the extraction electrodes 63 a and 63 b. Theelectrodes 3 b and 4 b of the respective resonators 3 and 4 areinterconnected with each other via the connection electrodes 34 and 44and the internal electrode 64, and are connected with the externalelectrode 71 via the extraction electrode 5 b disposed on one sidesurface of the base plate 5, and the internal electrode 61.

Although only one extraction electrode 5 b is provided on one sidesurface of the base plate 5, two extraction electrodes 5 b mayalternatively be provided on the two opposite side surfaces of the baseplate 5. This may contribute to a further prevention of disconnection ofthe conductive path.

Accordingly, a surface-mounted-chip acceleration sensor 1A is obtained.

FIG. 6 is a circuit diagram of an example of an acceleration sensordevice provided with the acceleration sensor 1A.

Such a sensor device utilizes separate oscillation effects of theacceleration-sensor element 2A. Specifically, the external electrodes 72and 71 of the acceleration sensor 1A are connected with an oscillationcircuit 9 a, and the external electrodes 73 and 71 are connected with anoscillation circuit 9 b. Each of the oscillation circuits 9 a and 9 bmay be, for example, the commonly known Colpitts oscillation circuit.The resonators 3 and 4 are separately oscillated by the respectiveoscillation circuits 9 a and 9 b. The oscillating frequencies f₁ and f₂are then input to a frequency-difference counter 9 c. Subsequently, thefrequency-difference counter 9 c outputs a signal V₀, which isproportional to the difference in the frequencies.

When acceleration G is applied to the acceleration sensor 1A, an inertiaforce acts upon the acceleration-sensor element 2A in a directionopposite to the direction in which the acceleration is applied. Thisbends the acceleration-sensor element 2A in the opposite direction tothe application direction of acceleration G. The bending of theacceleration-sensor element 2A generates stress, thus producing tensilestress in one of the resonators and compressive stress in the otherresonator. In the case where resonators of a thickness-shear vibrationtype are used, the oscillating frequency of the resonator with tensilestress decreases, whereas the oscillating frequency of the resonatorwith compressive stress increases. Accordingly, the difference in thefrequencies is obtained via the external electrodes 71, 72, and 73 sothat the signal V₀ proportional to the acceleration G can be obtained.

Using the acceleration sensor 1A in an environment where there is achange in temperature may lead to thermal expansion of the resonators 3and 4, the base plate 5, the casing components 6, and the covercomponents 7. If the coefficient of thermal expansion is different amongthe resonators 3 and 4 and the base plate 5, the acceleration-sensorelement 2A may bend when the temperature changes, thus generating stressin the resonators 3 and 4. This means that the difference in thefrequencies may change due to factors other than acceleration. On theother hand, if the resonators 3 and 4 and the base plate 5 are composedof materials having substantially the same coefficient of thermalexpansion, the same amount of stress will be generated in response to achange in temperature. Consequently, the outputs from the two resonators3 and 4 are received by the frequency-difference counter 9 c in adifferential manner, such that the changes in the output signals causedby, for example, a change in temperature affecting both resonators 3 and4 can be counterbalanced. Accordingly, an acceleration sensor devicehaving sensitivity that is solely reactive to acceleration G can beobtained.

On other hand, even if the coefficient of thermal expansion among theacceleration-sensor element 2A, the casing components 6, and the covercomponents 7 is different, a temperature change simply does not lead toa generation of stress in the acceleration-sensor element 2A since theacceleration-sensor element 2A is supported by these components only ina cantilever manner.

FIG. 7 illustrates another example of an acceleration sensor deviceprovided with the acceleration sensor 1A.

This acceleration sensor device utilizes a single oscillation effect ofthe acceleration-sensor element 2A. Specifically, the externalelectrodes 72 and 73 of the acceleration sensor 1A are connected with adifferential impedance sensor circuit 9 d, and the external electrode71, which is a common electrode, is connected with an oscillationcircuit 9 e. Moreover, reference numerals 9 f and 9 g indicate matchingresistors. In this case, both resonators 3 and 4 are oscillated with thesame frequency by the oscillation circuit 9 e, so that the phasedifference or the oscillation difference can be detected based on thedifference in electrical impedance between the resonators 3 and 4. Thus,the output V₀ proportional to acceleration G is obtained via thedifferential impedance sensor circuit 9 d. In order to achieveoscillation with the same frequency, the oscillator circuit 9 e may beformed in view of feedback on an output from one of the resonators or acombination of outputs from both resonators.

In this case, like the example shown in FIG. 6, a signal proportional toacceleration G can be obtained while also counterbalancing the changesin the outputs caused by, for example, a change in temperature.Accordingly, an acceleration sensor device having sensitivity that isonly reactive to acceleration G can be obtained.

Second Preferred Embodiment

FIGS. 8 and 9 illustrate an acceleration sensor according to a secondpreferred embodiment.

An acceleration sensor 1B includes a bimorph acceleration-sensor element2B held and enclosed by the casing components 6 and the cover components7, which are formed of, for example, insulative ceramics, in adouble-supported fashion. Components equivalent to those in the firstpreferred embodiment shown in FIGS. 1 to 4 are indicated by the samereference numerals, and descriptions of those components will thus beomitted.

The two longitudinal ends of the acceleration-sensor element 2B arefixedly supported by the pair of casing components 6 such that theacceleration-sensor element 2B horizontally intervenes the casingcomponents 6. The casing components 6 are preferably substantiallyU-shaped in cross section. Furthermore, the cover components 7 arerespectively attached to the upper and lower open planes.

The electrodes 3 a and 4 a of the resonators 3 and 4 are respectivelyconnected with internal electrodes 61 a and 61 b, which are disposed onfirst end-portions of the top surfaces of the casing components 6, viathe connection electrodes 33 and 43. On the other hand, the electrodes 3b and 4 b of the resonators 3 and 4 are connected with an internalelectrode 65, extending continuously across the top surface of thecombination of the acceleration-sensor element 2B and the casingcomponents 6, via the respective connection electrodes 34 and 44. Theinternal electrodes 61 a and 61 b are respectively connected with theexternal electrodes 72 and 73 disposed on the outer surface of the covercomponents 7, whereas the internal electrode 65 is connected with theexternal electrode 71.

When using the acceleration-sensor element 2B of a double-supportedstructure as in the second preferred embodiment, signals can be detectedfrom both longitudinal ends of the acceleration-sensor element 2B. Sucha structure allows easier extraction of electrodes in comparison withthe acceleration-sensor element 2A of a cantilever structure. Forexample, the extraction electrode 5 b disposed on a side surface of thebase plate 5 and the extraction electrodes 62 a and 63 a disposed oninner side surfaces of the casing components 6 can be omitted.Furthermore, the anisotropic electrically-conductive adhesive providedfor connecting the connection electrodes 33 and 43 with the respectiveextraction electrodes 62 a and 63 a can also be omitted.

The acceleration sensor according to the present invention is notlimited to the above-described preferred embodiments.

For example, although the resonators 3 and 4 used in the first andsecond preferred embodiments are preferably of a thickness-shearvibration type, resonators of other alternative vibration types (such asa thickness-extensional vibration type or a longitudinal vibration type)may be used.

Furthermore, although the base plate and each of the first and secondresonators in the above-described preferred embodiments have a gaptherebetween given a wider dimension than the range in which theresonator vibrates in an enclosed manner, the base plate and eachresonator may alternatively be joined with each other in an opposingmanner such that the surfaces of the base plate and the resonator areentirely attached to each other. Such an entirely-attached state maycause deterioration of the performance (Q and K) of the resonators sincethe base plate limits the vibration of the resonators, but is effectivein view of the efficiency for generating stress in response to theacceleration.

Although the present invention has been described and illustrated indetail with reference to certain preferred embodiments thereof, it isclearly understood that the same is by way of illustration and exampleonly and is not to be taken by way of limitation, the spirit and scopeof the present invention being limited only by the terms of the appendedclaims.

1. An acceleration sensor comprising: a base plate; and first and secondresonators each including a piezoelectric material and each havingelectrodes on two opposite main surfaces thereof, each of the first andsecond resonators having a vibrating section at an intermediate portionof the respective resonator with respect to the longitudinal directionthereof; wherein the first and second resonators are attached toopposite sides of the base plate with respect to a direction in whichacceleration is applied so as to define a bimorph acceleration-sensorelement, at least one longitudinal end of the acceleration-sensorelement is fixed such that the first and second resonators bend in thesame direction in response to the acceleration, and changes in frequencyor changes in impedance in the first and second resonators caused by thebending of the acceleration-sensor element are differentially detectedin order to detect the acceleration; the acceleration-sensor element isbendable about a central bending plane in response to the acceleration,the central bending plane being positioned at a central portion of thebase plate with respect to the application direction of acceleration;each of the first and second resonators is attached to the base platesuch that the opposite main surfaces having the electrodes disposedthereon are substantially perpendicular to the application direction ofacceleration; and a height of the first and second resonators in adirection that is substantially perpendicular to the applicationdirection of acceleration is smaller than a height of the base plate inthe direction that is substantially perpendicular to the applicationdirection of acceleration.
 2. The acceleration sensor according to claim1, wherein the first and second resonators are attached to the oppositesides of the base plate at positions where the first and secondresonators are opposed to each other.
 3. The acceleration sensoraccording to claim 2, wherein each of the first and second resonators isattached to the central portion of the base plate with respect to aheight direction of the base plate, the height direction beingsubstantially perpendicular to the application direction ofacceleration.
 4. The acceleration sensor according to claim 1, whereinthe base plate and the first and second resonators are made of at leastone material having substantially the same coefficient of thermalexpansion.
 5. The acceleration sensor according to claim 1, whereinopposite outer surfaces of the acceleration-sensor element arerespectively fixedly supported by a pair of casing components at saidlongitudinal end of the acceleration-sensor element, the outer surfacesbeing opposite to each other with respect to the application directionof acceleration, and open planes defined by the acceleration-sensorelement and the casing components are covered with a pair of covercomponents so that a displacement portion of the acceleration-sensorelement, which is bendable in response to the acceleration, is disposedwithin an enclosed space, one of the electrodes in each of the first andsecond resonators is disposed at a free-end side of the resonator and isconnected with a common electrode via an extraction electrode providedon the base plate, the common electrode being provided at a fixed-endside of an outer surface of a combination of the casing components andthe cover components, the other electrode in the first resonator isdisposed at a base-end side of the first resonator, said other electrodein the first resonator being connected with a first independentelectrode provided at a free-end side of the outer surface of thecombination of the casing components and the cover components, saidother electrode in the first resonator being connected with the firstindependent electrode via a first extraction electrode provided on oneof the casing components, and the other electrode in the secondresonator is disposed at a base-end side of the second resonator, saidother electrode in the second resonator being connected with a secondindependent electrode provided at the free-end side of the outer surfaceof the combination of the casing components and the cover components,said other electrode in the second resonator being connected with thesecond independent electrode via a second extraction electrode providedon the other casing component.
 6. The acceleration sensor according toclaim 5, wherein the casing components are provided with a plurality ofinternal electrodes disposed on upper surfaces of the casing components,the internal electrodes being connected with the electrodes in each ofthe first and second resonators.